Spray drying foamed material



United States Patent SPRAY DRYING FOAMED MATERIAL Ismar M. Reich, New York, and William R. Johnston,

Chappaqua, N. Y., assignors to Standard Brands Incorporated, New York, N. Y., a corporation of Delaware No Drawing. Application April 1, 1953, Serial No. 346,263

4 Claims. (Cl. 99-71) This invention relates to a. spray drying process which affords greatly improved control of dry product bulk density, particle size, and color. The process is applicable to drying any liquid which can be pumped, containing solid material in solution or suspension, and particularly to drying aqueous cofiee and tea extracts.

In the conventional commercial preparation of such products as instant coffee and instant tea the dry material is obtained by dehydration of an extract. This may be accomplished by spray drying, vacuum drum or belt drying, freeze drying, etc., but the most widely used method is spray drying.

Recent trends in the drying of coffee and tea extracts have been in the direction of increasing the particle size of the dried material. The reasons for the preference for a coarse granular product over a fine powder include:

1. Improved rate of solution by avoidance of the clumping caused by fine particles.

2. Attractive appearance.

3. Darker more coffee-like or tea-like color.

4. Cleaner, less dusty, more free-flowing characteristies.

In general, it is recognized in the spray drying art that the size of the dried particles depends on the size of the sprayed droplets. Methods of controlling the atomization to produce a coarse spray are well known. Other factors being constant, however, large droplets must require more time to dry than smaller ones. In spray drying there is always a limited amount of time available depending on the size and design of the drier. Particles striking the walls or floor of the drier before being sufliciently dry will adhere. If the air velocity is low, so that the particles fall freely, the time available depends on the height of the drier and is less for larger particles than for smaller ones. If the air velocity is great enough to carry the particles in the exhaust air, the time the particles are in the drier is roughly directly proportional to the drier volume and inversely proportional to the air flow rate. With excessive air flow the particles may be removed from the drying chamber and collected before completion of drying. Increasing the air temperature will increase the rate of drying but temperature increase must be limited because of its effect on product quality and because of danger of fire. Thus the production of spary dried products of large particle size presents serious problems, particularly where it is necessary to limit the size of the drier and hence limit the time available for drying.

Another problem in spray drying is control of bulk density of the dried material. This is espcially important inconsumer products where the product may have to be used by the spoonful. In packaging in a container of predetermined volume, slack fill and excessive overfill must be avoided. For particular drying equipment, the bulk density can normally be varied only slightly by small changes in conditions such as air temperature. Spray drier-s, then, are normally considered to be quite inflexible, particularly in regard to control of particle size and bulk density. i

2,788,276 Patented Apr. 9, 1957 It has now been discovered that by a combination of steps disclosed below it is possible to spray dry relatively very coarse particles, with bulk density and color shade controlled over wide ranges, in small standard commercial spray drying equipment. According to the invention, the solution or suspension to be dried is converted into a foam by the admixture of a gas therewith and this foam is then fed continuously to the drier in the usual manner and spray dried. The foam is a dispersion of small gas bubbles in the liquid, the liquid being the continuous phase and the gas the discontinuous phase. There is no apparent chemical reaction with the liquid extract although there may be some solution of gas in the extract.

It has been found that the bulk density of the dry product (weight per unit volume in the final packaged condition) is correlated with the density of the foamed extract which is spray dried. The foam density is in turn dependent upon the amount of gas incorporated in the foam; the more gas, the lower the foam density. The gas incorporated in the foam serves to increase the puffing effect which usually accompanies the atomization of liquids into an atmosphere of heated gas.

The bulk density of porous particulate materials must depend upon the intrinsic density of the solid material, the amount of porosity Within the individual particles, and also upon the shape and size distribution of the particles. In the invention, control of the amount of gas used to form the foam controls the bulk density by determining the porosity Within the particles. As explained below, a reduction in the foam density permits more rapid drying of large particles.

Typical spray dried hollow particles are characterized by a more or less spherical shape with a single empty space surrounded by a thick Wall. In contrast, the process of the invention produces particles Which have many internal spaces each enclosed by a thin wall. To a large extent the size and number of internal particle spaces can be easily controlled in practice. It has been found that strong shearing forces applied to the foam just before it is dried serve to reduce the size and increase the number of individual bubbles in the foam. This results in a larger number of smaller spaces Within the dried particles with correspondingly thinner walls surrounding each space although the particle size may be large. As a result of this state of increased subdivision of the dry material within the particles, light transmitted and reflected by them is scattered to a greater extent. This con dition has the effect of causing the product to appear lighter in shade just as the comminution of materials which transmit and reflect light generally makes them appear to be of a paler cast. Thus, by varying the bubble size in the foam, the intensity of color in the dry product may be controlled.

Another factor which affects the shade of the dry particles is the nature of the gas used to prepare the foam. For example, it has been demonstrated that carbon dioxide foam produces larger spaces in the particles and consequently a darker color than does nitrogen foam used in the same manner. Foams made with nitrogen are more stable than those made with carbon dioxide. Thus, under similar conditions, a carbon dioxide foam has a coarser structure and hence yields a product with a thicker internal Wall structure.

In order to increase the particle size of spray dried particles Without increasing the size of the drier it is necessary to minimize the time required for drying, which time, of course, depends on the rate of drying and the ra d r dsont ar a a a fo dr the a r 7 density.

' pressure atomiziug system.

temperature, and the coefiicient of heat transfer. If the diameter of the sprayed droplets is increased the area available for drying increases in proportion to the square of the diameter but the. amount or". Water to be evaporated increases as the cube of the diameter because of the increased weight of liquid in the larger particles. It

has been determined empirically and reported in the literature that the coefficient of heat transfer is inversely proportional to the diameter of droplets. The time required to dry a particle of given size will also depend upon the density of the particle which is proportional to the product bulk density if the particle shape and size distribution are not changed. Thus, in summary, the required drying timeis proportional to the square of the particle diameter, to the ratio of water to solids in the extract, and to the bulk density of the dry product. For example, should it be desired to double the average particle diameter without increasing the drying time, temperature being unchanged, this could be accomplished by adjusting the foam density to produce a product of onehalf the bulk'density and by halving the ratio of Water to solidsin the solution or suspension. The present invention provides a convenient means for adjusting the bulk Any gas which will .not react to the detriment of. the product, for instance, air, nitrogen and carbon dioxide in solid or gaseous form may be used to produce the foam.

. In the case of aqueous coffee extracts the coffee solids concentration is preferably in the range of to 70%. In the case of aqueous tea extracts the tea solids concennation is preferably in the range of to 75 The density of the foam may vary within wide limits. in the case of aqueous solutions it may be, for example, from 0.05 to 1.0 g./ml. and in the case of aqueous coffee extracts, preferably 0.4. to 0.7 g./rnl. In the case :of aqueous tea extracts'the foam densityis preferably 0.1 to 0.4 g./ml.

' Table it presents data on gas (carbon dioxide) to liquid ratios at 70 'C., by weightand by volume for various foam densities. The volumetric ratios are calculated at 1 atmosphere pressure, at which the foam density is measured, and at 275 atmospheres (4040 p. s. i. a.), which corresponds to a typical spray pressure. The calculations are also based on-a liquid extract having a density of 1.24 g./ml. at 70 C. I

TABLE .Curbon dioxide gas per unit li quid extract in foam at I a 70 C.

Foam Densityat 1' atm., Volumetric Volumetric Weight ml. Ratio, Ratio, 275 Ratio 1 atm 7 atm.

Q 1 Neglectlng'posstble solubility of gash: extract.

i neers Handbook, edited "by John H. Perry, 3rd edition,

Thi's'drier-is of mixed flow de- McG'raw-Hill "(1950). sign, i. e., the flow of spray and'drying gas is partly parallel and partly counter-current, and employs a high it is unusually efficient in terms of production'rate for its size. Calculation of the over-all average contact time available by dividing the drier volume by the air flow rate indicates a time of approximately four seconds.

I The coffee extract was pumped to a controlled pressure of '40 pounds per square inch and -a,controlled flow of carbon dioxide gas was introduced'into-the extract at this pressure. Since, it was desired in this instance to produce a dark colored product, carbon dioxide was used rather than air and the foam was not subjected to a great deal of shearing force. The combined flow of gas and liquid extract in the form of a foam, was then raised to a pressure of 150 pounds per square inch with a booster pump which fed a high pressure reciprocating pump in which the foam was compressed and raised to the 4000 pounds per square inch pressure employed for spraying. By regulation of the amount of carbon dioxide introduced the density of the foam at the high pressure pump measured at atmospheric pressure was adjusted to 0.5 g./ml. The bulk density of the dry product obtained was 0.25 g./ml. The product was relatively dark in color, of very coarse particle size and had excellent solubility in hot and cold Water.

Example lI.-A pure aqueous tea extract containing solids was converted into a foam and spray dried in the manner described in Example I. The tea extract was pumped to a controlled pressure of 650 pounds per square inch and a-controlled flow of carbon dioxide gas was introduced into the extract at this pressure. The combined flow of gas and liquid extract, in the form of a foam, was then fed directly to a high pressure reciproeating pump in which the foam was compressed and raised to the 4500 pounds per square inch pressure employed for spraying. The density of a foam sample withdrawn and measured at atmospheric pressure was about 0.25

'desired'spray pressure may be used. Furthermore, other types of atomizing devices in addition to pressure-type nozzles may be employed, such, for example, as twofluid type nozzles.

Since certain changesmay be made in the above process which embodies the invention without departing from its spirit or scope, itis intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense. It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described.

We claim:

1. In a process of spray drying aqueous extracts of tea and roasted coffee, the steps of dispersing a gas of the group consisting of'air, carbon dioxide and nitrogen in an aqueous extract of a substance of the group consisting of tea and roasted coffee to produce a foam with the liquid forming the continuous phase having a density in the range from about 0.05 to 1.0 g./ml. measured at atmospheric pressure and then spray drying the foam.

2. In a process of spray drying aqueous extracts of tea and roasted coffee, the steps of dispersing a gas of the group consisting of air, carbon dioxide and nitrogen in an aqueous extract of a'substance of the group consisting oftea and roasted coffee having asolids content'in the range of about-40% to 75% to produce a foam with the liquid forming the continuous phase having a'density in the range from about 0.05 to 1.0 g./ml. measured at atmospheric pressureandithen spray-drying the foam.

3. Ina process of'spray drying an aqueous extractof roasted coifee, the 'steps ofdispersing a gasof the group consisting of air, carbon dioxide and nitrogen in an aqueous extract-of-roasted coifee having a solids content in the range of about 40% to 70% to produce a foam with the liquid forming the continuous phase having a density in the range of about-0.4 r007 g./rn'l. measured at atmospheric pressure and then spraydrying the foam.

4. 111 a process of spray drying an aqueous tea extract, the steps of dispersing a gas of the groupconsisting' of air, carbon dioxide and nitrogen in an aqueous tea extract having a solids content in the range of about 45% to 75% to produce a foam with the liquid forming the continuous phase having a density in the range of about 0.1 to 0.4 g./m1. measured at atmospheric pressure and 5 then spray drying the foam.

References Cited in the file of this patent UNITED STATES PATENTS 1,406,381 Heath et a1. Feb. 14, 1922 10 

1. IN A PROCESS OF SPRAY DRYING AQUEOUS EXTRACTS OF TEA AND ROASTED COFFEE, THE STEPS OF DISPERSING A GAS OF THE GROUP CONSISTING OF AIR, CARBON DIOXIDE AND NITROGEN IN AN AQUEOUS EXTRACT OF A SUBSTANCE OF THE GROUP CONSISTING OF TEA AND ROASTED COFFEE TO PRODUCE A FOAM WITH THE LIQUID FORMING THE CONTINUOUS PHASE HAVING A DENSITY IN THE RANGE FROM ABOUT 0.05 TO 1.0 G/ML. MEASURED AT ATMOSPHERIC PRESSURE AND THEN SPRAY DRYING THE FOAM. 